I'll go first.
A dentist who must have guessed I was being abused by my then boyfriend and gave me very low cost treatments for his work. It was only years later I realised he must have been basically doing it at cost price because he knew there was no way I could live without the work but as a student had absolutely no spare money. His work genuinely made my life worth living again and 20+ years later it's still perfect. He will never know what a difference he made then and still to this day.
So tell me your stories of people who made a difference to you and they don't know it.